High-power electron sources (up to 500 kW) have conventionally been used in X-ray irradiation applications, including food irradiation and sterilization. Usually, a pencil beam of electrons is rastered, which includes scanning an area from side to side while a conveyance system translates the object to cover the irradiated object. The electrons traverse a thin window that separates the source vacuum from air. The window can be easily cooled to prevent rupture since it is thin and since the electron beam is rastered, it spreads the electron energy over a large area. Thus, it is easier to cool than heat concentrated in a small spot.
In typical X-ray radiography, electrons in a beam impinge upon a stationary target to generate X-rays. The target is usually tungsten-rhenium brazed with copper that is cooled with chilled circulating water to remove the heat deposited by the electrons. High-energy X-ray inspection systems typically employ sources up to 1 kW that may include the use of this type of target. There are, however, emerging inspection applications where there is a need to increase power to approximately 20 kW to allow for greater penetration and enable new technologies. However, at these higher powers, the heat from the target cannot be removed fast enough to the point of target liquefaction, thus destroying the target.
Medical X-ray tubes used in Computed Tomography (CT) applications require very high power (up to 100 kW) with sub-millimetric focal spots. FIG. 1 illustrates a typical rotating anode X-ray tube 100 used in medical applications. Glass envelope 102 encloses a cathode 104 comprising a filament 106 in a focusing cup, and an anode/target 108 coupled with a tungsten/rhenium, anode disk 110 via an anode stem 114. In order to prevent melting of the anode/target 108 in such tubes, the target 108 is rotated at very high speed (˜8,000 rpm) by using a motor comprising a rotor 109 and a stator 111, causing the heat within target 108 to dissipate over a large area. Since it is impractical to pass a rotating shaft through a high-vacuum seal, the rotating parts of the tube are positioned within the glass vacuum envelope 102 comprising a port 116 through which generated X-rays leave the tube 100. Temperature management is achieved by the heat storage capacity of the target 108. Since the heat removal by conduction is negligible and the heat storage capacity is limited, the tube 100 needs to be turned off for some time before turning it on again, thereby reducing the duty factor. Unlike medical applications, however, some security inspection systems require continuous operation. Hence, there is a need for a high-power X-ray source that can be operated continuously and that does not have issues with overheating.
Another method that has been used for high-power targets is based on a liquid metal target. FIG. 2 illustrates a typical liquid metal target assembly for use in an X-ray source. At least a portion of the target 202 is cooled by a circulating liquid metal 204. A heat exchanger 206 is used to cool down the liquid metal 204 and a pump 208 is used to recirculate the liquid metal 204. The liquid metal 204 serves as both the X-ray production target as well as the cooling fluid, in that the heat generated by an electron beam 210 hitting the target surface 202 is carried away by the flowing stream of liquid metal 204. The advantage of this method is that it allows for continuous operation as the liquid metal can be cooled fast enough.
Possible liquid metals include liquid Gallium, which has high thermal conductivity, high volume specific heat and low kinetic viscosity. However, Gallium has a low atomic number (Z) of 32 as compared to Tungsten (Z=74), which results in lower X-ray conversion efficiency and a narrower Bremsstrahlung fan angle. Mercury is a liquid metal at room temperature with a high Z (80), however it is not usually used for this application due to its hazardous nature. A suitable metal alloy consists of 62.5% Ga, 21.5% In and 16% Sn. However, the atomic number of the aforementioned alloy is also quite low as compared to Tungsten. Another suitable alloy may be composed of elements having a higher Z, such as of 43% Bi, 21.7% Pb, 18.3% In, 8% Sn, 5% Cd and 4% Hg. However, Mercury, Cadmium and Lead are all hazardous materials. Another disadvantage of the liquid metal targets is that they require a thin window to separate the vacuum from the liquid target. The probability of such window rupturing and contaminating the vacuum is high.
Therefore, there is a need for a high-power X-ray production target that can be cooled in a safe and effective manner. Further, an X-ray tube with such a target should be capable of operating in a continuous mode.